Early electrical heating elements for high temperature furnaces operating in a temperature range of 2500.degree. C. to 3000.degree. C. include donut shaped heating elements which consume inordininately large amounts of electric urrent and produce uneven and inefficient heating. A major improvement to high temperature furnaces was the cylindrical hanging heating element which was invented by Charles Hill in 1960. The hanging heating element includes a number of hanging panels, such as three, which are made of tungsten or molybdenum and are supported at their upper ends by rods or bars which also function as connectors for applying electric current to the panels. The lower ends of the panels are interconnected by a thick metal ring which serves as a common electrical connector and is able to expand and contract freely during furnace operation with no mechanical constraints. The metal ring has conventionally been made thick and heavy to handle the large alternating current, on the order of 2000 amperes, which flows through the panels and ring.
The panels may be in the form of thin sheets, or may be formed of a mesh as disclosed in U.S. Pat. No. 3,469,013, to James S. Hetherington et al.
There has been little improvement in hanging heating elements since 1967 due to static business conditions with unchanging requirements. However, hanging heating elements have become quite large in recent years, posing new problems.
The larger heating panels are made of molybdenum or tungsten in the form or 10 to 15 mil thick sheets. Although the sheets are originally quite ductile, they recrystallize after a short period of operation in excess of 1200.degree. C., becoming extremely brittle and fragile. In addition, the metal at the re-entrant angle where the panels join the ring becomes embrittled due to welding and riveting operations performed during the fabrication process.
Installation of conventional hanging heating elements is difficult and hazardous since six heavy, fragile panels which are connected only at their lower ends must be simultaneously manipulated into position. The heating chamber is usually at high location such as 6 feet above floor level and it is difficult to manipulate and align the element within the insulation lined housing. Movement of any one of the panels away from the center can create a micro-crack at the re-entrant angle which propagates through the panel, ending its life prematurely. With the panels permanently connected to the lower ring, damage to one panel requires replacement of the entire heating element, which is very expensive and time consuming. Similarly, failure of one panel during operation requires replacement of the entire element.